Recent Progress in Hormone Research 57:235-256 (2002)
© 2002 The Endocrine Society
Neuroendocrine Modulation and Repercussions of Female Reproductive Aging
Phyllis M. Wise,
Matthew J. Smith,
Dena B. Dubal,
Melinda E. Wilson,
Shane W. Rau,
Adrienne B. Cashion,
Martina Böttner and
Katherine L. Rosewell
Department of Physiology, University of Kentucky College of Medicine, Lexington, Kentucky 40536-0298
 |
ABSTRACT
|
|---|
The menopause marks the end of a woman’s reproductive life. During the postmenopausal period, plasma estrogen concentrations decrease dramatically and remain low for the rest of her life, unless she chooses to take hormone replacement therapy. During the past 20 years, we have learned that changes in the central nervous system are associated with and may influence the timing of the menopause in women. Recently, it has become clear that estrogens act on more than just the hypothalamus, pituitary, ovary, and other reproductive organs. In fact, they play roles in a wide variety of nonreproductive functions. With the increasing life span of humans from approximately 50 to 80 years and the relatively fixed age of the menopause, a larger number of women will spend over one third of their lives in the postmenopausal state. It is not surprising that interest has increased in factors that govern the timing of the menopause and the repercussions of the lack of estrogen on multiple aspects of women’s health. We have used animal models to better understand the complex interactions between the ovary and the brain that lead to the menopause and the repercussions of the hypoestrogenic state. Our results show that when rats reach middle age, the patterns and synchrony of multiple neurochemical events that are critical to the preovulatory gonadotropin-releasing hormone (GnRH) surge undergo subtle changes. The precision of rhythmic pattern of neurotransmitter dynamics depends on the presence of estradiol. Responsiveness to this hormone decreases in middle-aged rats. The lack of precision in the coordination in the output of neural signals leads to a delay and attenuation of the luteinizing hormone surge, which lead to irregular estrous cyclicity and, ultimately, to the cessation of reproductive cycles. We also have examined the impact of the lack of estrogen on the vulnerability of the brain to injury. Our work establishes that the absence of estradiol increases the extent of cell death after stroke-like injury and that treatment with low physiological levels of estradiol are profoundly neuroprotective. We have begun to explore the cellular and molecular mechanisms that underlie this novel nonreproductive action of estrogens. In summary, our studies show that age-related changes in the ability of estradiol to coordinate the neuroendocrine events that lead to regular preovulatory GnRH surges contribute to the onset of irregular estrous cycles and eventually to acyclicity. Furthermore, we have shown that the lack of estradiol increases the vulnerability of the brain to injury and neurodegeneration.
|
I. Introduction
|
|---|
Women undergo the menopause at approximately 51 years of age. The timing of this dramatic physiological change has remained essentially constant since records have been kept. The menopause occurs at the time of the exhaustion of the ovarian follicular reserve. Since the ovarian follicles are not only the source of germ cells but also are the primary source of estrogens, plasma estrogen concentrations drop precipitously during the postmenopausal years and remain low for the remainder of a woman’s life, unless she chooses to take hormone replacement therapy (HRT). Thus, the end of the reproductive life has far-reaching implications for women because they become permanently hypoestrogenic at this time. In recent years, we have come to appreciate that estrogens are not only reproductive hormones but also pleiotropic hormones that play roles in a wide variety of nonreproductive functions as disparate as bone and mineral metabolism (Lindsay, 1996; Manolagas, 2000; Compston, 2001), memory and cognition (Fillit, 1994; Erkkola, 1996; Sherwin, 1996,1999), cardiovascular function (Wild, 1996; Matthews et al., 2000; Mendelsohn, 2000; Stevenson, 2000; Losordo and Isner, 2001), and the immune system (Jansson and Holmdahl, 1998; Ahmed and Hissong, 1999). With the substantial increase in the average life span of humans from approximately 50 to 80 years that occurred during the last century, and the relatively fixed age of the menopause, the number of women who will spend over one third of their lives in the postmenopausal state has increased dramatically. It is not surprising, then, that an increasing number of clinical and basic science researchers have focused their efforts on understanding reproductive aging, since it has become increasingly important to understand the factors that drive the menopausal transition. A number of excellent reviews that discuss the role of the brain in female reproductive aging have appeared during the past 10 years (Lu et al., 1994; Wise et al., 1997; Rubin, 2000; Lapolt and Lu, 2001). This review focuses primarily on our work; however, we have benefited immensely from the studies of our colleagues and will cite them in appropriate places. It should be noted that a better understanding of the mechanisms regulating female reproductive aging will be important to gerontologists because the female reproductive system undergoes senescence relatively early during the aging process, in the absence of pathological changes that often confound gerontological studies. Therefore, we hope that concepts derived from our understanding of the aging reproductive system may shed light on the process of the biology of aging of other systems (Kirkwood, 1998).
|
II. The Central Nervous System Plays an Important Role in Female Reproductive Aging
|
|---|
Our studies have focused on the influence of changes in the central nervous system (CNS) on female reproductive aging and the repercussions of hypoestrogenicity on the brain. For many years, it was accepted that reproductive aging - and, in particular, the menopause in women - resulted simply from the depletion of the postmitotic pool of ovarian follicles that is set down during embryonic development (vom Saal et al., 1994). It was thought that CNS changes that accompany the menopause were merely a consequence of declining ovarian function. More recently, investigators have realized that the brain plays an important role in the sequence of events leading to reproductive senescence. It appears that the temporal patterns of neural signals undergo subtle but important changes during middle age in both women and animal models, before the cessation of reproductive cycles, and that these changes may accelerate the loss of follicles leading to the menopause.
These conclusions are based predominantly upon studies performed in rodent models. Therefore, one must ask whether experimental results using animal models of reproductive senescence will improve our understanding of the human menopause. This remains controversial, with advocates on both sides of this lively discussion. On the one hand, since rodents do not undergo a real menstrual cycle, by definition, they do not undergo a true ‘meno-pause.’ Arguments that rodents are not good models of the human menopause are based on two important observations. First, in postmenopausal women, plasma gonadotropin levels are high; whereas in aged acyclic, repeatedly pseudopregnant rats, they remain relatively normal, despite substantial decreases in estradiol in both species (for a review, see Wise, 2000). This suggests that decreased estrogen secretion is paramount to the postmenopausal state and hypothalamic influences may not be as critical in the human female. In contrast, hypothalamic influences are paramount to the postreproductive state in rodents: ovaries of young rodents transplanted into old hosts fail to cycle normally, whereas ovaries of old rodents transplanted into young hosts respond to neuroendocrine signals and reproductive cyclicity is restored. However, the regularity of the cycle is not totally restored (Peng and Huang, 1972; Felicio et al., 1983). Second, it has been suggested that the temporal dynamics of loss of the ovarian follicular reserve are fundamentally different in women, compared to rodents. However, we would contend that thorough study and careful analysis of the rate of loss of primordial follicles have not been performed in rodents the way they have been in humans (Faddy et al., 1983; Richardson et al., 1987).
Despite these differences between older postmenopausal women and older acyclic rats, striking parallels exists between middle-aged female rats and pre- and perimenopausal women. First, the sentinel event that heralds impending reproductive decline in both humans and rats is a rise in follicle-stimulating hormone (FSH) concentrations (DePaolo, 1987; Klein et al., 1996). In humans, the change is prominent during the periovulatory phase of the menstrual cycle. In a similar manner, middle-aged rats exhibit elevated FSH levels during estrous afternoon. Second, the pattern of luteinizing hormone (LH) secretion changes in both perimenopausal women and middle-aged rats as they enter the transition to acyclicity. Controversy exists as to the changes in pulsatile LH release in pre- and perimenopausal women. A report by Matt et al. (1998) shows that, in regularly cycling, middle-aged women, the duration of LH pulses increases and the frequency of pulses decreases. To our knowledge, the work of Matt and colleagues is the only study performed in regularly cycling, premenopausal women. None of the other studies in humans have controlled for changing cycle length during the perimenopausal period when they monitored LH pulses. This may underlie the discrepancy among studies in humans. These results mirror the changes that we observed in middle-aged, regularly cycling rats (Scarbrough and Wise, 1990). Third, menstrual cycle length in women and estrous cycle length in rats become highly variable (Sherman et al., 1976; Fitzgerald et al., 1994). Cycles of increased and decreased length have been reported in women between the ages of 37–45 years, as they enter the perimenopausal transition. Likewise, rats exhibit highly variable estrous cycles, with prolonged periods of estrus or diestrus between each preovulatory LH surge. Fourth, although the postmenopausal period is characterized by extremely low levels of estradiol, several studies (Klein et al., 1996; Santoro et al., 1996) have shown that estradiol concentrations do not decrease during the pre- and perimenopausal period but, in fact, remain normal or are elevated. These new findings are strikingly similar to what has been observed in middle-aged rats as they enter the transition to irregular cyclicity (Butcher and Page, 1981; Lu, 1983). Finally, the ability of estradiol to induce LH surges is attenuated in both perimenopausal women and middle-aged rats. Van Look and colleagues (1977) showed that estradiol was able to induce LH surges of attenuated amplitude in only a small portion of the women studied. This parallels precisely the changes that we observed in middle-aged rats (Wise, 1984). For all these reasons, we believe that rodents serve as excellent models in which to examine the factors that initiate the process of reproductive aging during middle age. We assume that information gained from these species can be extrapolated to humans and will allow us to uncover and explore concepts that can be generalized to human reproductive aging.
|
III. Changes in the Pattern of Gonadotropin Secretion Reflect Changes in Hypothalamic GnRH Input
|
|---|
We (Wise, 1982a) and other investigators (van der Schoot, 1976; Lu, 1983; Nass et al., 1984) established that one of the earliest changes that occurs during middle age is in the secretion pattern of the preovulatory LH and FSH surge. In rats that had not exhibited any change in estrous cycle length and still maintained normal, regular, 4-day estrous cycles, we observed a consistent delay in the onset of the LH surge and attenuation in peak concentrations (Figure 1). These changes occur at an age when we could not detect any changes in the responsiveness of the pituitary gland to GnRH. Therefore, we asked: do GnRH neurons change during the middle-aged period, leading to alterations in the ability to drive LH secretion? Methods to directly quantitate changes in the secretion pattern of GnRH have been problematic, since the hormone is not detectable in peripheral blood, GnRH neurons are few in number, and are distributed diffusely in the anterior preoptic region of the hypothalamus. Instead, investigators have used immunocytochemical methods to determine the number of GnRH neurons and the percentage that are activated. Expression of immediate early genes (e.g., Fos) by individual neurons has been used as a marker of increased neuronal activity (Hoffman et al., 1993). Therefore, we used this method to test whether alterations in the timing and amplitude of the proestrous LH surge involve alterations in the activation of GnRH neurons. In these studies, we have never detected any age-related change in the number of GnRH neurons (Lloyd et al., 1994; Krajnak et al., 2001). However, the number of GnRH neurons that express Fos during the proestrous LH surge decreased dramatically in middle-aged, regularly cycling rats (Figure 2). These results have been confirmed and extended using three-dimensional reconstructions of the forebrain populations of GnRH neurons in young and middle-aged rats (Rubin et al., 1995). Our studies strongly suggest that the pattern of afferent input to GnRH neurons changes during the early stages of reproductive aging. It should be noted that additional changes in GnRH activity have been reported by investigators who have utilized rats that have reached more advanced stages of reproductive senescence, particularly during times of increased demand on the population of GnRH neurons. These changes include decreased in vivo output of GnRH in conjunction with the steroid-induced LH surge using push-pull perfusion (Rubin and Bridges, 1989) and decreased LH pulse frequency and amplitude in ovariectomized rats (Scarbrough and Wise, 1990). In addition, investigators have observed a decline in pituitary GnRH receptor mRNA (Rubin and Jimenez-Linan, 1999) and binding (Marchetti and Cioni, 1988), which are regulated by the pattern of pulsatile GnRH secretion. These age-related receptor changes are likely to lead to decreased responsiveness to GnRH (Smith et al., 1982; Hogg et al., 1992; Brito et al., 1994).
View larger version (19K):
[in this window]
[in a new window]
|
FIG. 1. The luteinizing hormone (LH) surge is blunted and delayed in middle-aged, compared to young, rats. Young and middle-aged regularly cycling rats were sequentially bled from right atrial cannulae during the day of proestrus. Plasma was radioimmunoassayed for LH. The first significant increase in LH was delayed by 1 hour and attenuated significantly in middle-aged rats.
|
|
View larger version (19K):
[in this window]
[in a new window]
|
FIG. 2. Percent of gonadotropin-releasing hormone (GnRH) neurons that express Fos during the proestrous LH surge decreases with age. Young and middle-aged rats regularly cycling rats were perfused with paraformaldehyde and their brains sectioned for dual immunocytochemical localization of GnRH and Fos. Age significantly decreased the level of activation of GnRH neurons. [Reprinted with permission from Lloyd JM, Hoffman GE, Wise PM 1994 Decline in immediate early gene expression in gonadotropin-releasing hormone neurons during proestrus in regularly cycling, middle-aged rats. Endocrinology 134:1800–1805. Copyright The Endocrine Society.]
|
|
|
IV. Changes in the Temporal Pattern and Synchrony of Neurotransmitter Input May Alter the Pattern of GnRH Secretion
|
|---|
The pattern of preovulatory GnRH secretion is determined by afferent input from multiple neurotransmitters and neuropeptides. Over the past 50 years, researchers have come to recognize the complexity of the signaling system that ultimately leads to the GnRH surge. We continue to add to the list of neurotransmitters and neuropeptides that modulate the pattern of GnRH secretion. The hierarchy of modulators is still unclear. Indeed, it is not clear whether a single neurotransmitter is essential or whether there is plasticity in the repertoire of neurotransmitters that can participate or substitute for one another in the generation of the GnRH surge. Data favor the latter possibility: there is remarkable redundancy and plasticity in the ensemble of factors that influences GnRH secretion. Together, they insure the maintenance or reappearance of LH surges when one of the players is disrupted. Thus, Clifton and Sawyer (1979) found that disruption of the catecholaminergic input to GnRH neurons halts estrous cyclicity but only temporarily, suggesting that, when necessary, other inputs are able to replace the important role that catecholamines normally play in GnRH secretion. It is clear that the synchrony, timing, and interplay among the multiple neural signals are required to insure that the proper timing and amplitude of preovulatory LH surges is maintained.
In an elegant and seminal series of studies beginning over 50 years ago, Everett and Sawyer (Everett et al., 1949; Sawyer et al., 1949; Everett and Sawyer, 1950,1953) showed that if the neural signals that regulate the LH surge are delayed by even 2 hours, the surge is delayed by an entire day, occurring at the proper time 24 hours later. Thus, these studies implicated that a circadian pacemaker regulates the precise timing of the events leading to the LH surge and the exact timing of the surge itself. Thus, the daily rhythmicity in the activity of neural events serves as a foundation for the orderly timing of the GnRH surge and hence the preovulatory release of LH. Over the past several years, we have examined whether the diurnal rhythm in various aspects of neural activity is altered in middle-aged rats as they begin the transition to reproductive senescence. We measured the diurnal rhythm of monoamine turnover rates (Wise, 1982b; Cohen and Wise, 1988), neurotransmitter receptor densities (Weiland and Wise, 1990), and neuropeptide mRNAs (Weiland et al., 1992; Cai and Wise, 1996; Krajnak et al., 1998; M.J. Smith, A.B. Cashion, L. Jennes and P.M. Wise, unpublished observations). The clear theme that emerges is that virtually all of the rhythms are dampened or altered when female rats are middle aged and begin the transition to irregular estrous cycles. Often, the attenuation in rhythmicity was progressive and changes were more exaggerated as animals aged. The change in rhythmicity of any single neurotransmitter must be considered subtle, since the overall average often did not change. Investigators who measure these endpoints at only one time of day would be unlikely to detect a significant age-related change. Yet, together, disruption of the synchrony and coordination of multiple neural signals that govern the precise timing of GnRH secretion may ultimately lead to profound changes in the ability of rats to maintain regular reproductive cyclicity.
Until very recently, we thought that deterioration in the integrity of the circadian biological clock, which is located in the suprachiasmatic nuclei (SCN) in mammals, may underlie the desynchronization of multiple rhythms (Wise et al., 1988,1997). However, our newest findings suggest that we must modify our thinking. We tested our hypothesis by measuring the rhythm of key neuropeptides in the SCN. We reasoned that if aging involves a change in the integrity of the SCN itself and all its essential elements (i.e., inputs, oscillators, outputs), the pattern of expression of all critical neuropeptides of the SCN would be affected. On the other hand, if only some of the components of the clock are affected with age, we might observe differential effects on these neuropeptides. We measured the rhythm of gene expression of two key functionally critical neuropeptides heavily expressed in the SCN: vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP) (Krajnak et al., 1998). VIP neurons are located primarily in the ventrolateral aspect of the SCN, where they receive direct retinal input (Ibata et al., 1989). The rhythmic expression of VIP mRNA and protein depends upon exposure to the light/dark cycle (Albers et al., 1990). Therefore, this neuropeptide is likely to convey time-of-day information to efferent targets in different regions of the brain. In contrast, AVP neurons are predominantly located in the dorsomedial portion of the SCN. The 24-hour rhythm in its gene and peptide expression is endogenous and does not depend upon the light/dark cycle for its existence. The AVP rhythm, therefore, serves as a marker of the integrity of the SCN (Gillette and Reppert, 1987). Both VIP and AVP neurons relay circadian information to several regions of the brain by sending efferent projections to diverse regions of the brain, including the rostral preoptic area, where they may influence GnRH neurons or other neurotransmitters that regulate GnRH secretion (Harney et al., 1996; van der Beek et al., 1999; Smith et al., 2000; Krajnak et al., 2001). Our results were clear - and surprising. As expected, VIP and AVP mRNA levels exhibited a 24-hour rhythm in young females. Furthermore, the rhythm in VIP mRNA disappeared by the time animals were middle aged (Figure 3). In marked contrast, the AVP mRNA rhythm was totally unaffected with age: the rhythm and overall level of mRNA were the same in young, middle-aged, and old rats, both in terms of the amount of mRNA/cell (Figure 4) and the number of cells expressing AVP mRNA. This was the first time that we had observed the preservation of a neural rhythm in middle-aged rats. Based on these results, we concluded that the integrity of the entire biological clock does not deteriorate in a unified manner; instead, age differentially influences various components of the SCN.
View larger version (17K):
[in this window]
[in a new window]
|
FIG. 3. VIP mRNA levels/cell in young and middle-aged ovariectomized, estradiol-treated rats as measured by in situ hybridization exhibits age-related changes in rhythmicity. Young, middle-aged, and old rats were killed at 7 times of day over a 24-hour period. Young rats exhibited a diurnal rhythm in gene expression. The rhythm was no longer detectable in middle-aged or old rats. [Reprinted from Krajnak K, Kashon ML, Rosewell KL, Wise PM 1998 Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA, but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats. J Neurosci 18:4767–4774.]
|
|
View larger version (21K):
[in this window]
[in a new window]
|
FIG. 4. Arginine vasopressin (AVP) mRNA levels/cell in young and middle-aged ovariectomized, estradiol-treated rats as measured by in situ hybridization exhibits age-related changes in rhythmicity. Brain sections from the same young, middle-aged, and old rats were used to measure AVP and vasoactive intestinal peptide (VIP) (see Figure 3). The diurnal rhythm was identical in all age groups. [Reprinted from Krajnak K, Kashon ML, Rosewell KL, Wise PM 1998 Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA, but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats. J Neurosci 18:4767–4774.]
|
|
The disappearance of the rhythm in VIP expression in the SCN is particularly intriguing, since this neuropeptide may play a uniquely important role in conveying time-of-day information directly to GnRH neurons. We hypothesized that the decrease in VIP levels in the SCN and the disappearance of its rhythm may lead to the delay in the timing of the LH surge and the attenuation in its amplitude. To test this possibility, we suppressed the level and rhythm of VIP by administering antisense oligonucleotides to VIP directed at the SCN of ovariectomized, estradiol-treated rats and assessed the effect on the LH surge (Harney et al., 1996). Peak LH concentrations during the surge were delayed and attenuated in antisense-treated animals, compared to random, oligo-treated control rats, in a manner that was strikingly similar to that observed previously in middle-aged rats (Figure 5). Similar results have been obtained using administration of VIP antisera intracerebroventricularly to ovariectomized, estradiol-treated rats (van der Beek et al., 1999). More recently, we determined whether aging alters the innervation of GnRH neurons by VIP and/or the ability of VIP to activate GnRH neurons by examining the effects of aging on the number of GnRH neurons apposed by VIP fibers and the number of GnRH neurons that receive VIP input that express Fos. Using triple-label immunocytochemistry for GnRH, VIP, and Fos in young and and middle-aged females, we quantified the number of GnRH neurons, GnRH neurons apposed by VIP fibers, and the number of GnRH neurons that express Fos that are apposed by VIP fibers. Our results clearly demonstrate that aging does not alter the number of GnRH neurons that receive VIP innervation. However, the number of GnRH neurons that receive VIP innervation and co-express Fos decreases significantly (Krajnak et al., 2001) (Figure 6). Immunocytochemical methods and light microscopy did not allow us to evaluate whether VIP acted directly upon GnRH neurons or through another neurotransmitter. However, several lines of evidence suggest that the communication between VIP and GnRH neurons is direct. 1) Using triple-label immunofluorescence to simultaneously localize GnRH, VIP, and VIP2 receptor protein, we showed that about 40% of all GnRH neurons analyzed contain VIP2 receptor immunoreactivity and that VIP-containing processes were seen in close apposition to a significant number of VIP2 receptor-positive GnRH neurons (Smith et al., 2000). 2) Horvath and colleagues demonstrated synaptic contacts between VIP- and GnRH-containing neurons (Horvath et al., 1998). 3) Lesions of the SCN indicate the presence of direct, VIP-containing projections to GnRH neurons (van der Beek et al., 1993). Together, these findings provide further support for a direct, VIP-containing pathway from the SCN to GnRH neurons and indicate that VIP can communicate directly with GnRH neurons. Furthermore, it appears that the age-related delay in the timing of the LH surge is not due to a change in VIP innervation of GnRH neurons but instead may result from a decreased sensitivity of GnRH neurons to VIP input.
View larger version (18K):
[in this window]
[in a new window]
|
FIG. 5. The steroid-induced LH is blunted and delayed in rats that were treated with antisense oligos to VIP or control scrambled oligos directly at the suprachiasmatic nucleus (SCN). Ovariectomized, estradiol-treated young rats were administered antisense or scrambled oligos and sequentially bled. The steroid-induced surge of LH exhibited changes that are remarkably like those observed during aging. [Reprinted with permission from Harney JP, Scarbrough K, Rosewell KL, Wise PM 1996 In vivo antisense antagonism of vasoactive intestinal peptide in the suprachiasmatic nucleus causes aging-like changes in the estradiol-induced LH and prolactin surge. Endocrinology 137:3696–3701. Copyright The Endocrine Society.]
|
|
View larger version (24K):
[in this window]
[in a new window]
|
FIG. 6. (A) Number of GnRH-immunopositive neurons per section; (B) percent of GnRH and VIP-immunopositive neurons; (C) percent of GnRH and Fos-immunopositive neurons; and (D) percent of GnRH, Fos, and VIP immunoreactive neurons in the preoptic area of young and middle-aged females during the peak of a steroid-induced LH surge exhibit age-related changes. Aging is associated with no change in the number of GnRH immunopositive neurons or the percent of GnRH and VIP immunopositive neurons. However, percent of activated GnRH neurons and the percent of activated GnRH that were closely apposed to VIP neurons decreased with age. [Reprinted from Krajnak K, Rosewell KL, Wise PM 2001 Fos-induction in gonadotropin-releasing hormone neurons receiving vasoactive intestinal polypeptide innervation is reduced in middle-aged female rats. Biol Reprod 64:1160–1164.]
|
|
|
V. Aging Influences the Balance Between Stimulatory and Inhibitory Neural Inputs to GnRH Neurons
|
|---|
The ultimate pattern of GnRH release is governed by the orchestration of stimulatory and inhibitory inputs. Most work has focused on stimulatory side of the balance sheet, since, clearly, this is critical to the GnRH surge. Most recently, investigators have focused attention on the possible role of decreased glutamate input to GnRH neurons in aging rats (Zuo et al., 1996; Gore et al., 2000a,b), since glutamate is a neurotransmitter that exerts important direct stimulatory effects on GnRH neurons (Brann, 1995; Eyigor and Jennes, 1996). However, we are beginning to appreciate more deeply that the amplitude and timing of the preovulatory LH surge depends upon a decrease in inhibitory tone (Akabori and Barraclough, 1986a,b; Smith and Gallo, 1997). Opioid peptides and gamma aminobutyric acid (GABA) are critical inhibitory neurotransmitters that normally restrain GnRH secretion during the estrous cycle. A decrease in their activity normally occurs on proestrous afternoon, permitting stimulatory factors to maximally influence GnRH neurons. Researchers have found that, unless the inhibitory inputs to GnRH neurons are suppressed, the effects of norepinephrine and other stimulatory factors do not result in LH surges of normal amplitude or timing (for a review, see Kalra and Kalra, 1984). In addition, pharmacological blockade of the inhibitory tone early on proestrus results in a premature LH surge. We currently are examining the diurnal rhythm of preprodynorphin mRNA levels in young and middle-aged proestrous rats to determine whether the rhythm of this neuropeptide is altered and whether its activity increases in middle-aged rats and may contribute to the diminished LH surge (M.J. Smith, A.B. Cashion, L. Jennes, and P.M. Wise, unpublished observations). Preliminary data (Figure 7) demonstrate that, in young proestrous rats, preprodynorphin mRNA levels decrease prior to the LH surge, decreasing inhibitory tone and possibly allowing stimulatory factors to be maximally effective. In middle-aged rats, the diurnal rhythm is no longer detectable and overall mRNA levels are higher, compared to young controls. Thus, it is possible that the inhibitory tone that dynorphin communicates to GnRH neurons is amplified in middle-aged rats and does not subside to allow stimulatory factors to act. This could contribute to the attenuated and delayed preovulatory surge observed in middle-aged animals. We currently are examining whether GABAergic tone also may increase with age. This is another important inhibitory neuropeptide that communicates directly with GnRH neurons (Petersen et al., 1993) and exerts an important inhibitory tone on GnRH secretion. The role of this inhibitory neurotransmitter may be particularly important, since estradiol’s actions on GABA may regulate cyclic morphological changes in astrocytes (Tranque et al., 1987; Parducz et al., 1993; Mong et al., 1999). These changes in the stellation of astrocytes on proestrus may then affect ensheathment of neurons, including possibly GnRH neurons (Cashion et al., 2001), leading to changes in the ability of neurotransmitters to communicate with each other to coordinate the GnRH surge.
View larger version (18K):
[in this window]
[in a new window]
|
FIG. 7. Preprodynorphin mRNA levels are higher and lack rhythmicity in middle-aged compared to young rats on proestrus. Young and middle-aged rats were killed at 0300 and 1200h on proestrus. Brains were prepared for in situ hybridization. Preprodynorphin mRNA was analyzed in the anteroventral periventricular nucleus (AVPV). Overall levels of gene expression are elevated and the rhythm is no longer detectable by the time rats reach middle age.
|
|
|
VI. Estradiol Is a Neuroprotective Factor
|
|---|
Recently, many researchers have focused their attention on the nonreproductive protective actions of estrogen. Aging and the menopause involve the gradual depletion of the ovarian follicular reserve and, with it, a decrease in plasma levels of estradiol. Since women will be spending a considerable portion of their lives in a hypoestrogenic state, the potential that women will be more vulnerable to neurodegenerative diseases and injury, due to the lack of estrogen, becomes even more important to understand. Numerous recent reviews provide excellent documentation of the many experimental and clinical circumstances in which estrogens provide profound protection against neuronal cell death (Green and Simpkins, 2000; Hurn and Macrae, 2000; Roof and Hall, 2000; Garcia Segura et al., 2001; Wise et al., 2001a,b). We will focus on our studies in this emerging area of interest.
We reported that low physiological levels of estradiol replacement dramatically decrease the degree of brain injury and cell death in an animal model of cerebrovascular stroke (Dubal et al., 1998). When we occlude the middle cerebral artery (MCAO) and permanently decrease blood flow to approximately 50% of normal, both the cerebral cortex and striatum undergo cell death. Ovariectomized rats are particularly vulnerable to injury and exhibit progressive infarction that evolves over a 24-hour period. Replacement with estradiol, to levels that mimic those which normally occur during the estrous cycle, results in profound protection of the cortex but not the striatum. Interestingly, middle-aged rats were equally responsive to the protective actions of estradiol (Dubal and Wise, 2000) (Figure 8). Our findings that estradiol replacement exerts equivalent neuroprotection in young and middle-aged female rats were unexpected because responsiveness of the hypothalamus to estradiol, as measured by a variety of endpoints - such as estradiol-induced activation of GnRH neurons that leads to LH surges (Wise, 1982b,1984; Lloyd et al., 1994), organization of diurnal rhythmicity in the hypothalamic neurotransmitter activity (Wise, 1982b; Cohen and Wise, 1988) or gene expression (Weiland et al., 1992; Krajnak et al., 1998), and stimulation of progesterone receptor binding (Wise et al., 1984) - diminishes with age. Therefore, we had hypothesized that estradiol would be less able to protect the brains of older animals against ischemic brain injury.
View larger version (17K):
[in this window]
[in a new window]
|
FIG. 8. Estradiol protects against middle cerebral artery occlusion (MCAO) in young and middle-aged rats. Low and high physiological levels of estradiol decreased total injury as measured by staining of brain sections with 2% triphenyltetrazolium chloride and measurement of infarct size using a computer-assisted imaging system and NIH Image. [Reprinted with permission from Dubal DB, Wise PM 2000 Neuroprotective effects of estradiol in middle-aged female rats. Endocrinology 142:43–48. Copyright The Endocrine Society.]
|
|
We have begun to investigate the cellular and molecular mechanisms that mediate the protective actions of estradiol. Several of our observations lead us to believe that low physiological levels of estradiol act through estrogen receptor alpha (ER
)-dependent mechanisms, leading to changes in gene expression that favor cell survival and suppress apoptotic cell death. First, we found that estradiol slows the progression, rate, and extent of cell death in the brain. Hormone treatment does not influence the extent of cell death that occurs immediately after stroke injury. Instead, its effects are confined to protecting against delayed cell death that occurs during the later phases of injury. Thus, it appears that estradiol protects specifically against apoptotic cell death (Dubal et al., 2001a; Rau et al., 2001) but does not protect against immediate necrotic cell death. Second, we (Dubal et al., 1999a) reported that, within 24 hours of MCAO, ER mRNA is dramatically upregulated and estradiol pretreatment prevents injury-induced downregulation of ERß in the cerebral cortex (Figure 9). These data suggest that brain injury may influence responsiveness of the injured cerebral cortex to estradiol and induce differential actions that are mediated by each receptor subtype (Nilsen et al., 2000; Patrone et al., 2000). It is important to note that ER is not usually detectable in the cerebral cortex of the adult rat and is only transiently expressed in this brain region during neonatal development when the cortex undergoes dramatic neurogenesis, neuritogenesis, and differentiation. It is intriguing to speculate that the dramatic reappearance of ER in the cerebral cortex may allow a recapitulation of the developmental actions of estradiol in promoting neurogenesis and re-differentiation of the cortex. Several studies support the concept that, following stroke injury, specific features of brain function revert to those seen during early stages of development, with the process of recovery recapitulating ontogeny (reviewed in Cramer and Chopp, 2000). To test whether ER is a critical functional link in estradiol-mediated neuroprotection, we performed parallel studies in ER knockout mice (Dubal et al., 2001b). We found that deletion of ER completely abolishes the protective actions of estradiol in all regions of the brain, whereas estradiol’s ability to protect against brain injury is totally preserved in the absence of ERß (Figure 10). These results firmly establish ER as the critical mechanistic link that mediates the neuroprotective effects of physiological levels of estradiol. Finally, we have found that estradiol treatment alters the expression of multiple genes that have been implicated in the balance between cell survival and cell death, including bcl-2 (Dubal et al., 1999a), c-fos (Rau et al., 2000), galanin (Dubal et al., 1999b), and activin (Böttner et al., 2001). Whether any or all of these genes are critical functional mediators of estradiol’s protective actions is not clear at present. Much more work must be done to understand the roles of each of these factors in protecting the brain against injury and cell death.
View larger version (16K):
[in this window]
[in a new window]
|
FIG. 10. Estradiol protects against MCAO in wild-type mice of both genetic backgrounds and in ERßKO mice but not in ERKO mice. (Left panel) Estradiol significantly decreased infarct volume in wild-type compared with oil (vehicle)-treated controls. In contrast, in ERKO mice, estradiol did not exert any protective effect. (Right panel) Estradiol significantly decreased infarct volume in wild-type and ERßKO mice. Brain sections were stained with hematoylin and eosin and the volume of the infarct was quantified with a computer-assisted imaging system using NIH Image. [Reprinted from Dubal DB, Zhu B, Yu B, Rau SW, Shughrue PJ, Merchenthaler I, Kindy MS, Wise PM 2001 Estrogen receptor-, not -ß, is a critical link in estradiol-mediated protection against brain injury. Proc Natl Acad Sci USA 98:1952–1957. Copyright National Academy of Sciences.]
|
|
We have used in vitro methods to assess the protective actions of estradiol because this methodology provides invaluable tools that complement in vivo approaches. Multiple manipulations can be performed in vitro that are not technically or financially feasible using in vivo models. In particular, organotypic explants provide a powerful way to manipulate cellular environments in vitro, while maintaining interneurons, spatial relationships, local synaptic connections, and interactions with the local glial environment. Using explant cultures of the neonatal cerebral cortex, we (Wilson et al., 2000) have shown that low concentrations of estradiol protect against cell death. Our studies strongly suggest that ERs are critical, since the protection was not observed using 17-estradiol and was blocked by co-incubation with ICI 182,780, an ER antagonist (Figure 11). These findings complement those of Gollapudi and Oblinger (1999a,b), who showed that PC12 cells transfected with full-length rat ER respond to the protective effects of estradiol, whereas cells transfected with vector DNA alone are not protected by estradiol. Other investigators have found that pharmacological levels of estradiol protect, even in the absence of the ER (for reviews, see Green and Simpkins, 2000; Hurn and Macrae, 2000; Roof and Hall, 2000; Garcia Segura et al., 2001; Wise et al., 2001a,b). The cellular and molecular mechanisms that underlie these receptor-independent protective actions are likely to result from estrogen’s antioxidant, scavenging, immune-suppressing, and vascular actions. Together, these studies emphasize the breadth of the repertoire of mechanisms that estrogens use to protect against injury and cell death.
View larger version (31K):
[in this window]
[in a new window]
|
FIG. 11. Estradiol protects against ischemic injury-induced cell death in explant cultures of the cerebral cortex. Cell death, as measured by lactose dehydrogenase (LDH) release, is significantly decreased when explants were cultured in the presence of 17ß-estradiol. However, 17-estradiol failed to protect and ICI182,780, an ER antagonist, blocked the protective actions of 17ß-estradiol. [Reprinted from Wilson ME, Dubal DB, Wise PM 2000 Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors. Brain Res 873:235–242.]
|
|
|
VII. Summary
|
|---|
In summary, our understanding of the role of the brain in reproductive aging - and, conversely, the impact of reproductive aging on the brain - has increased dramatically during the past 20 years. Subtle changes in hypothalamic function and the ability of estradiol to influence the secretion of GnRH begin early during the aging process, ultimately leading to reproductive acyclicity. We are increasingly aware that the permanent hypoestrogenic state has major repercussions on multiple organs and physiological systems. Much attention is focused on the possible mechanisms through which estrogens may act as protective factors.
|
REFERENCES
|
|---|
Ahmed, Hissong SA
1999 Gender and risk of autoimmune diseases: possible role of estrogenic compounds.
Envir Health Perspect Supp
107:
681
Akabori A, Barraclough CA
1986a Effects of morphine on luteinizing hormone secretion and catecholamine turnover in the hypothalamus of estrogen-treated rats.
Brain Res
362:
221–226[CrossRef][Medline]
Akabori A, Barraclough CA
1986b Gonadotropin responses to naloxone may depend upon spontaneous activity in noradrenergic neurons at the time of treatment.
Brain Res
362:
55–62[CrossRef][Medline]
Albers HE, Stopa EG, Zoeller RT, Kauer JS, King JC, Fink JS, Mobtaker H, Wolfe H
1990 Day-night variation in prepro vasoactive intestinal peptide/peptide histidine isoleucine mRNA within the rat suprachiasmatic nucleus.
Mol Brain Res
7:
85–89[Medline]
Böttner M, Dubal DB, Rau SW, Wise PM
2001 Activin gene expression increases after stroke injury: modulation by estradiol.
Soc Neurosci Abstr
27:
1165
Brann DW
1995 Glutamate: a major excitatory transmitter in neuroendocrine regulation.
Neuroendocrinology
61:
213–225[Medline]
Brito AN, Sayles TE, Krieg RJ Jr, Matt DW
1994 Relation of attenuated proestrous luteinizing hormone surges in middle-aged female rats to in vitro pituitary gonadotropin-releasing hormone responsiveness.
Eur J Endocrinol
130:
540–544[Abstract]
Butcher RL, Page RD
1981 Role of the aging ovary in cessation of reproduction. In Schwartz NB, Hunzicker-Dunn M, eds.
Dynamics of Ovarian Function. New York: Raven Press;
253–271
Cai A, Wise PM
1996 Age-related changes in the diurnal rhythm of CRH gene expression in the paraventricular nuclei.
Am J Physiol
270:
E238–E243[Abstract/Free Full Text]
Cashion AB, Smith MJ, Wise PM
2001 Astrocyte morphology in the rostral preoptic area (rPOA) exhibits a diurnal rhythm on proestrus: relationship to the LH surge and effects of age.
Soc Neurosci Abstr
27:
1940
Clifton DK, Sawyer CH
1979 LH release and ovulation in the rat following depletion of hypothalamic norepinephrine: chronic vs. acute effects.
Neuroendocrinology
28:
442–449[Medline]
Cohen IR, Wise PM
1988 Age-related changes in the diurnal rhythm of serotonin turnover in microdissected brain areas of estradiol-treated ovariectomized rats.
Endocrinology
122:
2626–2633[Abstract]
Compston JE
2001 Sex steroids and bone.
Physiol Rev
81:
419–447[Abstract/Free Full Text]
Cramer SC, Chopp M
2000 Recovery recapitulates ontogeny.
Trends Neurosci
23:
265–271[CrossRef][Medline]
DePaolo LV
1987 Age-associated increases in serum follicle-stimulating hormone levels on estrus are accompanied by a reduction in the ovarian secretion of inhibin.
Exptl Aging Res
13:
3–7
Dubal DB, Wise PM
2000 Neuroprotective effects of estradiol in middle-aged female rats.
Endocrinology
142:
43–48[Abstract/Free Full Text]
Dubal DB, Kashon ML, Pettigrew LC, Ren JM, Finklestein SP, Rau SW, Wise PM
1998 Estradiol protects against ischemic injury.
J Cereb Blood Flow Metab
18:
1253–1258[CrossRef][Medline]
Dubal DB, Shughrue PJ, Wilson ME, Merchenthaler I, Wise PM
1999a Estradiol modulates Bcl-2 in cerebral ischemia: a potential role for estrogen receptors.
J Neurosci
19:
6385–6393[Abstract/Free Full Text]
Dubal DB, Wilson, ME, Shughrue PJ, Merchenthaler I, Wise PM
1999b Induction of galanin gene expression in estradiol-mediated neuroprotection against cerebral ischemia.
Soc Neurosci Abstr
25:
1449
Dubal DB, Rau SW, Wise PM
2001a Estradiol protects against delayed cell death in stroke injury.
Soc Neurosci Abstr
27:
1164
Dubal DB, Zhu B, Yu B, Rau SW, Shughrue PJ, Merchenthaler I, Kindy MS, Wise PM
2001b Estrogen receptor-, not -ß, is a critical link in estradiol-mediated protection against brain injury.
Proc Natl Acad Sci USA
98:
1952–1957[Abstract/Free Full Text]
Erkkola R
1996 Female menopause, hormone replacement therapy, and cognitive processes.
Maturitas
23:
S27–S30
Everett JW, Sawyer CH
1950 A 24-hour periodicity in the ‘LH-release apparatus’ of female rats, disclosed by barbiturate sedation.
Endocrinology
47:
198–218
Everett JW, Sawyer CH
1953 Estimated duration of the spontaneous activation which causes release of ovulating hormone from the rat hypophysis.
Endocrinology
52:
83–92
Everett JW, Sawyer CH, Markee JE
1949 A neurogenic timing factor in control of the ovulatory discharge of luteinizing hormone in the cyclic rat.
Endocrinology
44:
234–250[Medline]
Eyigor O, Jennes L
1996 Identification of glutamate receptor subtype mRNAs in gonadotropin-releasing hormone neurons in rat brain.
Endocrine
4:
133–139
Faddy MJ, Gosden RG, Edwards RG
1983 Ovarian follicle dynamics in mice: a comparative study of three inbred strains and an F1 hybrid.
J Endocrinol
96:
23–33[Medline]
Felicio LS, Nelson JF, Gosden RG, Finch CE
1983 Restoration of ovulatory cycles by young ovarian grafts in aging mice: potentiation by long-term ovariectomy decreases with age.
Proc Natl Acad Sci USA
80:
6076–6080[Abstract/Free Full Text]
Fillit H
1994 Estrogens in the pathogenesis and treatment of Alzheimer’s disease in postmenopausal women.
Ann NY Acad Sci
743:
233–239[Medline]
Fitzgerald CT, Seif MW, Killick SR, Bennett DA
1994 Age-related changes in the female reproductive cycle.
Br J Obstet Gynaecol
101:
229–223[Medline]
Garcia Segura LM, Azcoitia I, DonCarlos LL
2001 Neuroprotection by estradiol.
Progr Neurobiol
63:
29–60[CrossRef][Medline]
Gillette MU, Reppert SM
1987 The hypothalamic suprachiasmatic nuclei: circadian patterns of vasopressin secretion and neuronal activity in vitro.
Brain Res Bull
19:
135–139[CrossRef][Medline]
Gollapudi L, Oblinger MM
1999a Estrogen and NGF synergistically protect terminally differentiated, ER-transfected PC12 cells from apoptosis.
J Neurosci Res
56:
471–481[CrossRef][Medline]
Gollapudi L, Oblinger MM
1999b Stable transfection of PC12 cells with estrogen receptor (ER): protective effects of estrogen on cell survival after serum deprivation.
J Neurosci Res
56:
99–108[CrossRef][Medline]
Gore AC, Oung T, Yung S, Flagg RA, Woller MJ
2000a Neuroendocrine mechanisms for reproductive senescence in the female rat: gonadotropin-releasing hormone neurons.
Endocrine
13:
315–323[CrossRef][Medline]
Gore AC, Yeung G, Morrison JH, Oung T
2000b Neuroendocrine aging in the female rat: the changing relationship of hypothalamic GnRH neurons and NMDA receptors.
Endocrinology
141:
4757–4767[Abstract/Free Full Text]
Green PS, Simpkins JW
2000 Neuroprotective effects of estrogens: potential mechanisms of action.
Intl J Dev Neurosci
18:
347–358[CrossRef][Medline]
Harney JP, Scarbrough K, Rosewell KL, Wise PM
1996 In vivo antisense antagonism of vasoactive intestinal peptide in the suprachiasmatic nucleus causes aging-like changes in the estradiol-induced LH and prolactin surge.
Endocrinology
137:
3696–3701[Abstract]
Hoffman GE, Smith MS, Verbalis JG
1993 c-Fos and related immediate early gene products as markers of activity in neuroendocrine systems.
Front Neuroendocrinol
14:
173–213[CrossRef][Medline]
Hogg BB, Matt DW, Sayles TE
1992 Attenuated proestrous luteinizing hormone surges in middle-aged rats are associated with decreased pituitary luteinizing hormone-beta messenger ribonucleic acid expression.
Am J Obstet Gynecol
167:
303–307[Medline]
Horvath TL, Cela V, van der Beek EM
1998 Gender-specific apposition between vasoactive intestinal peptide-containing and gonadotropin-reeasing hormone producing neurons in the rat.
Brain Res
795:
277–281[CrossRef][Medline]
Hurn PD, Macrae IM
2000 Estrogen as a neuroprotectant in stroke.
J Cereb Blood Flow Metab
20:
631–652[CrossRef][Medline]
Ibata Y, Yakahaski Y, Okamura H, Kawakami F, Terubayashi H, Kubo T, Yanaihara N
1989 Vasoactive intestinal peptide (VIP)-like immunoreactive neurons located in the rat suprachiasmatic nucleus receive a direct retinal projection.
Neurosci Lett
97:
1–5[CrossRef][Medline]
Jansson L, Holmdahl R
1998 Estrogen-mediated immunosuppression in autoimmune diseases.
Inflamm Res
47:
290–301[CrossRef][Medline]
Kalra SP, Kalra PS
1984 Opioid-adrenergic-steroid connection in regulation of luteinizing hormone secretion in the rat.
Neuroendocrinology
38:
418–426[Medline]
Kirkwood TBL
1998 Ovarian ageing and the general biology of senescence.
Maturitas
30:
105–111[CrossRef][Medline]
Klein NA, Illingworth PJ, Groome NP, McNeilly AS, Battaglia DE, Soules MR
1996 Decreased inhibin B secretion is associated with the monotropic FSH rise in older, ovulatory women: a study of serum and follicular fluid levels of dimeric inhibin A and B in spontaneous menstrual cycles.
J Clin Endocrinol Metab
81:
2742–2745[Abstract]
Krajnak K, Kashon ML, Rosewell KL, Wise PM
1998 Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA, but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats.
J Neurosci
18:
4767–4774[Abstract/Free Full Text]
Krajnak K, Rosewell KL, Wise PM
2001 Fos-induction in gonadotropin-releasing hormone neurons receiving vasoactive intestinal polypeptide innervation is reduced in middle-aged female rats.
Biol Reprod
64:
1160–1164[Abstract/Free Full Text]
Lapolt PS, Lu JKH
2001 Factors influencing the onset of female reproductive senescence. In Hof PR, Mobbs CV, eds.
Functional Neurobiology of Aging. San Diego: Academic Press;
761–768
Lindsay R
1996 The menopause and osteoporosis.
Obstet Gynecol
87:
16S–19S
Lloyd JM, Hoffman GE, Wise PM
1994 Decline in immediate early gene expression in gonadotropin-releasing hormone neurons during proestrus in regularly cycling, middle-aged rats.
Endocrinology
134:
1800–1805[Abstract]
Losordo DW, Isner JM
2001 Estrogen and angiogenesis.
Arterioscler Thromb Vasc Biol
21:
6–12[Abstract/Free Full Text]
Lu JK, Anzalone CK, Lapolt PS
1994 Relation of neuroendocrine function to reproductive decline during aging in the female rat.
Neurobiol Aging
15:
541–544[CrossRef][Medline]
Lu JKH
1983 Changes in ovarian function and gonadotropin and prolactin secretion in aging female rats. In Meites J, ed.
Neuroendocrinology of Aging. New York: Plenum Press;
103–122
Manolagas SC
2000 Cellular and molecular mechanisms of postmenopausal osteoporosis. In Bellino FL, ed.
Proceedings of the International Symposium on the Biology of the Menopause. Norwell, MA: Springer;
134–146
Marchetti B, Cioni M
1988 Opposite changes of pituitary and ovarian receptors for LHRH in ageing rats: further evidence for a direct neural control of ovarian LHRH receptor activity.
Neuroendocrinology
48:
242–251[Medline]
Matt DW, Kauma SW, Pincus SM, Veldhuis JD, Evans WS
1998 Characteristics of luteinizing hormone secretion in younger versus older premenopausal women.
Am J Obstet Gynecol
178:
504–510[CrossRef][Medline]
Matthews KA, Kuller LH, Sutton-Tyrrell K
2000 Changes in cardiovascular risk factors during the peri- and postmenopausal years. In Bellino FL, ed.
Proceedings of the International Symposium on the Biology of the Menopause. Norwell, MA: Springer;
147–158
Mendelsohn ME
2000 Mechanisms of estrogen action in the cardiovascular system.
J Steroid Biochem Molec Biol
74:
337–343[CrossRef][Medline]
Mong JA, Glaser E, McCarthy MM
1999 Gonadal steroids promote glial differentiation and alter neuronal morphology in the developing hypothalamus in a regionally specific manner.
J Neurosci
19:
1464–1472[Abstract/Free Full Text]
Nass TE, Lapolt PS, Judd HL, Lu JKH
1984 Alterations in ovarian steroid and gonadotrophin secretion preceding the cessation of regular oestrous cycles in ageing female rats.
J Endocrinol
100:
43–50[Abstract]
Nilsen J, Mor G, Naftolin F
2000 Estrogen-regulated developmental neuronal apoptosis is determined by estrogen receptor subtype and the fas/fas ligand system.
J Neurobiol
43:
64–78[CrossRef][Medline]
Parducz A, Perez J, Garcia-Segura LM
1993 Estradiol induces plasticity of GABAergic synapses in the hypothalamus.
Neuroscience
53:
395–401[CrossRef][Medline]
Patrone C, Pollio G, Vegeto E, Enmark E, de Curtis I, Gustafsson J-A, Maggi A
2000 Estradiol induces differential neuronal phenotypes by activating estrogen receptor or ß.
Endocrinology
141:
1839–1845[Abstract/Free Full Text]
Peng M-T, Huang H-H
1972 Aging of hypothalamic-pituitary-ovarian function in the rat.
Fertil Steril
23:
535–542[Medline]
Petersen SL, McCrone S, Coy D, Adelman JP, Mahan LC
1993 GABAA receptor subunit mRNAs in cells of the preoptic area: colocalization with LHRH mRNA using dual-label in situ hybridization histochemistry.
Endocr J
1:
29–34
Rau SW, Dubal DB, Wise PM
2000 Immediate early gene regulation by estradiol in ischemic injury.
Soc Neurosci Abstr
26:
778
Rau SW, Dubal DB, Wise PM
2001 Estradiol protects against apoptotic cell death in stroke injury: possible mechanisms.
Soc Neurosci Abstr
27:
1165
Richardson SJ, Senikas V, Nelson JF
1987 Follicular depletion during the menopausal transition: evidence for accelerated loss and ultimate exhaustion.
J Clin Endocrinol Metab
65:
1231–123.[Abstract]
Roof RL, Hall ED
2000 Gender differences in acute CNS trauma and stroke: neuroprotective effects of estrogen and progesterone.
J Neurotrauma
17:
367–388[Medline]
Rubin BS
2000 Hypothalamic alterations and reproductive aging in female rats: evidence of altered LHRH neuronal function.
Biol Reprod
63:
968–976[Abstract/Free Full Text]
Rubin BS, Bridges RS
1989 Alterations in luteinizing hormone-releasing hormone release from the mediobasal hypothalamus of ovariectomized, steroid-primed middle-aged rats as measured by push-pull perfusion.
Neuroendocrinology
49:
225–232[Medline]
Rubin BS, Jimenez-Linan M
1999 LHRH receptor mRNA levels and gonadotropin subunit mRNA levels are reduced in the pituitaries of middle-aged rats on the day of a steroid-induced LH surge.
Soc Neurosci Abstr
25:
1960
Rubin BS, Mitchell S, Lee CE, King JC
1995 Reconstructions of populations of LHRH neurons in young and middle-aged rats reveal progressive increases in subgroups expressing fos protein on proestrus and age-related deficits.
Endocrinology
136:
3823–3830[Abstract]
Santoro N, Brown JR, Adel T, Skurnick JH
1996 Characterization of reproductive hormonal dynamics in the perimenopause.
J Clin Endocrinol Metab
81:
1495–1501[Abstract]
Sawyer CH, Everett JW, Markee JE
1949 A neural factor in the mechanism by which estrogen induces the release of luteinizing hormone in the rat.
Endocrinology
44:
218–233[Medline]
Scarbrough K, Wise PM
1990 Age-related changes in the pulsatile pattern of LH release precede the transition to estrous acyclicity and depend upon estrous cycle history.
Endocrinology
126:
884–890[Abstract]
Sherman BM, West JH, Korenman SG
1976 The menopausal transition: analysis of LH, FSH, estradiol, and progesterone concentrations during menstrual cycles of older women.
J Clin Endocrinol Metab
42:
629–636[Abstract]
Sherwin BB
1996 Hormones, mood, and cognitive functioning in postmenopausal women.
Obstet Gynecol
87:
20S–26S
Sherwin BB
1999 Can estrogen keep you smart? Evidence from clinical studies.
J Psychiat Neurosci
24:
315–321[Medline]
Smith MJ, Gallo RV
1997 The effect of blockade of 
-opioid receptors in the medial preoptic area on the luteinizing hormone surge in the proestrous rat.
Brain Res
768:
111–119[CrossRef][Medline]
Smith MJ, Jennes L, Wise PM
2000 Localization of the VIP2 receptor protein on GnRH neurons in the female rat.
Endocrinology
141:
4317–4319[Abstract/Free Full Text]
Smith WA, Cooper RL, Conn PM
1982 Altered pituitary responsiveness to gonadotropin-releasing hormone in middle-aged rats with 4-day estrous cycles.
Endocrinology
111:
1843–1848[Abstract]
Stevenson JC
2000 Cardiovascular effects of oestrogens.
J Steroid Biochem Molec Biol
74:
387–393[CrossRef][Medline]
Tranque PA, Suarez I, Olmos G, Fernandez B, Garcia-Segura LM
1987 Estradiol-induced redistribution of glial fibrillary acidic protein immunoreactivity in the rat brain.
Brain Res
406:
348–351[CrossRef][Medline]
van der Beek EM, Wiegant VM, van der Donk HA, van den Hurk R, Buijs RM
1993 Lesions of the suprachiasmatic nucleus indicate the presence of a direct vasoactive intestinal polypeptide-containing projection to gonadotropin-releasing hormone neurons in the female rat.
J Neuroendocrinol
5:
137–144[Medline]
van der Beek EM, Swarts HJ, Wiegant VM
1999 Central administration of antiserum to vasoactive intestinal peptide delays and reduces luteinizing hormone and prolactin surges in estrogen-treated rats.
Neuroendocrinology
69:
227–237[CrossRef][Medline]
van der Schoot P
1976 Changing pro-oestrous surges of luteinizing hormone in ageing 5-day cyclic rats.
J Endocrinol
69:
287–288[Medline]
Van Look PFA, Lothian H, Hunter WM, Michie EA, Baird DT
1977 Hypothalamic-pituitary-ovarian function in perimenopausal women.
Clin Endocrinol
7:
13–31[Medline]
vom Saal FS, Finch CE, Nelson JF
1994 Natural history and mechanisms of reproductive aging in humans, laboratory rodents, and other selected vertebrates. In Knobil E, Neill JD, eds.
The Physiology of Reproduction. New York: Raven Press;
1213–1314
Weiland NG, Wise PM
1990 Aging progressively decreases the densities and alters the diurnal rhythm of alpha-1-adrenergic receptors in selected hypothalamic regions.
Endocrinology
126:
2392–2397[Abstract]
Weiland NG, Scarbrough K, Wise PM
1992 Aging abolishes the estradiol-induced suppression and diurnal rhythm of proopiomelanocortin gene expression in the arcuate nucleus.
Endocrinology
131:
2959–2964[Abstract]
Wild RA
1996 Estrogen: effects on the cardiovascular tree.
Obstet Gynecol
87:
27S–35S
Wilson ME, Dubal DB, Wise PM
2000 Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors.
Brain Res
873:
235–242[CrossRef][Medline]
Wise PM
1982a Alterations in proestrous LH, FSH, and prolactin surges in middle-aged rats.
Proc Soc Exp Biol Med
169:
348–354[Medline]
Wise PM
1982b Norepinephrine and dopamine activity in microdissected brain areas of the middle-aged and young rat on proestru
TOP
ABSTRACT
I. Introduction
